The evolution of car safety systems has gone from the simple lap seatbelt, followed by shoulder belts, to airbags and other active systems. In particular, the effectiveness of seatbelts can be improved by tightening the seatbelts with automatic seatbelt retractors in the early stages of a crash. As the forces produced by the crash push the vehicle occupants against the seatbelts, systems are available which allow the seatbelt webbing to extend to mitigate maximum crash loads. Such mitigation of maximum loads can result in higher survival rates and reduced injuries. Known mechanisms are relatively simple and involve stretching of the seatbelt webbing and the use of energy absorbing structure such as the shear shaft contained within the seatbelt webbing take-up spool. Modern methods of design, simulations, and crash testing have allowed the development of computer models and databases that can be used as design tools. These design tools are used to predict the injury which a vehicle occupant might be expected to suffer in a crash of a given magnitude with the safety systems of a particular design. Typically, the designer works with the parameters of available safety systems to try and optimize their functionality during a crash. However, a vehicle crash by its nature is to some extent an unpredictable event. The loads to which a vehicle's occupants are subjected vary dramatically between different crash scenarios. The designer's objective is to build a system which functions best in the majority of cases.  
Typically the designer can specify various parameters of the safety system, such as the amount the seatbelts are retracted, the size and energy dissipation profile of energy absorbing mechanisms which allow the seatbelts to extend under load, the velocity, placement, and the timing and logic of airbag deployment. However, once selected, the options for control during a particular crash are usually limited to deciding whether or not to deploy various systems based on sensor data. So, for example, if the crash is below a certain threshold the airbags may not be deployed, or the seatbelt retractors may not be activated. Recently airbags have been given the capability of varying the force with which they deploy, sometimes by having two stages, sometimes by having venting systems. Such capabilities allow the safety system to respond to the situation created by a particular crash as opposed to designing a system to respond generally to all possible crashes. What is needed are energy management devices for seatbelts that can be continually adjusted as a crash event is occurring. 